Several people have vision impairments associated with refractive properties of the eye, such as myopia (near-sightedness), hyperopia (far-sightedness) and astigmatism. Myopia occurs when light focuses before the retina, and hyperopia occurs when light is refracted to a focus behind the retina. Astigmatism occurs when the corneal curvature is unequal in two or more directions. These vision impairments can be corrected with spectacles or contact lenses. Alternatively, the cornea of the eye can be reshaped surgically to provide the needed optical correction.
Eye surgery has now become commonplace with some patients pursuing it as an elective procedure to avoid using contact lenses or glasses to correct refractive problems, and others pursuing it to correct adverse conditions such as cataracts. And, with recent developments in laser technology, laser surgery is becoming the technique of choice for ophthalmic procedures. The reason eye surgeons prefer a surgical laser beam over manual tools like microkeratomes and forceps is that the laser beam can be focused precisely on extremely small amounts of ocular tissue, thereby enhancing accuracy and reliability of the procedure. These in turn enable better wound healing and recovery following surgery.
Examples of surgically cutting eye tissues include cutting the cornea and/or the crystalline lens of the eye. The lens of the eye can be cut to remove a defect, such as a cataract. Other eye tissues, e.g. the cornea or the lens capsule may be cut to access the cataractous lens so it can be removed.
The cornea can also be cut and reshaped to correct a refractive error of the eye, for example with laser assisted in situ keratomileusis (“LASIK”), photorefractive keratectomy (“PRK”), radial keratotomy (“RK”), cornealplasty, astigmatic keratotomy, corneal relaxing incision (“CRT”), Limbal Relaxing Incision (“LRI”), and refractive lenticular extractions, such as small incision lenticular extractions, and flapless refractive lenticular extractions. With astigmatic keratotomy, corneal relaxing incisions, and limbal relaxing incisions, the corneal cuts are made in a well-defined manner and depth to allow the cornea to change shape and become more spherical.
Different laser eye surgical systems use different types of laser beams for the various procedures and indications. These include, for instance, ultraviolet lasers, infrared lasers, and near-infrared, ultra-short pulsed lasers. Ultra-short pulsed lasers emit radiation with pulse durations as short as 10 femtoseconds and as long as 3 nanoseconds, and a wavelength between 300 nm and 3000 nm. Examples of laser systems that provide ultra-short pulsed laser beams include Abbott Medical Optics' iFS Advanced Femtosecond Laser, Abbott Medical Optics' IntraLase FS Laser, and OptiMedica's Catalys Precision Laser System.
In the commonly-known LASIK procedure, an ultra-short pulsed laser is used to cut a corneal flap to expose the corneal stroma for photoablation with ultraviolet beams from an excimer laser. Photoablation of the corneal stroma with the excimer laser reshapes the cornea and corrects the refractive condition such as myopia, hyperopia, astigmatism, and the like.
Cataract extraction is also a frequently performed surgical procedure with an estimated 15 million cataract surgeries performed per year worldwide. Opacification of the natural crystalline lens of the lens leads to cataract formation. The cataract scatters light passing through the lens, thereby perceptibly degrading vision. A cataract can vary in degree from slight to complete opacity. Early in the development of an age-related cataract, the power of the lens may increase, causing near-sightedness (myopia). Gradual yellowing and opacification of the lens may reduce the perception of blue colors as those shorter wavelengths are more strongly absorbed and scattered within the cataractous crystalline lens. Often, cataract formation progresses slowly, resulting in progressive vision loss.
Typically, cataract treatment involves replacing the opaque crystalline lens with an artificial intraocular lens (IOL). Cataract surgery can be performed using a technique called phacoemulsification, in which an ultrasonic tip with associated irrigation and aspiration ports is used to sculpt the relatively hard nucleus of the lens to facilitate its removal through an opening made in the anterior lens capsule. The outer membrane of the lens, referred to as the lens capsule, contains the nucleus of the lens, which is often the site of the highest grade of the cataract.
Performing an anterior capsulotomy or capsulorhexis in which a small round hole is formed in the anterior side of the lens capsule provides access to the lens nucleus. When a laser is used to cut the lens capsule, the procedure is called capsulotomy, whereas when forceps and other manual surgical tools are used to tear the lens capsule, the procedure is called a manual continuous curvilinear capsulorhexis (CCC). After the capsulotomy, the laser may be used to segment the cataractous lens to ease its removal from the eye. After removal of the lens nucleus, a synthetic foldable intraocular lens (IOL) can be inserted into the remaining lens capsule of the eye.
Conventional ultra-short pulse laser systems have been used to cut eye tissue, and to treat many patients with cataracts. Sometimes, however, these systems may provide less than ideal results for treatment of at least some patients' eyes. This may occur because the eye comprises complex optical structures, making the success of laser eye surgery dependent on the accurate and precise measurement of both the position of the eye in connection with laser eye surgery system, as well as the measurement and/or imaging of the eye structures themselves. For example, in some instances, misalignment of the eye with the surgical treatment apparatus may result in less than ideal placement of incisions.
Other factors that may limit the usefulness of data provided to a surgical laser system from eye measurement devices, such as tomography and topography systems. For example, there can be at least some distortion of at least some of the images taken among different devices, and this distortion can make the placement of laser incisions less than ideal in at least some instances. Also, the use of different systems for measurement and treatment can introduce alignment errors, may take more time than would be ideal, and may increase the overall cost of surgery so that fewer patients receive beneficial treatments.
Another factor that may affect the accuracy of positioning and eye structure measurement is the occurrence of blinking. Blinking is the semi-autonomic rapid closing and opening of the eyelid. A patient may reflexively blink to protect the eye from perceived potential damage, or may do spontaneously, generally at rate of 10 to 15 times a minute. Each blink lasts for 100-400 milliseconds, during which it obstructs all pattern vision and attenuates light levels 100-folds. In addition, the reflection, refraction, and/or scattering of light from the eye lid is vastly different from the reflection, refraction, and/or scattering of light off surfaces of the eye, such as the cornea. As a result, data on eye measurement and eye position based on the reflective, refractive or other properties of the eye may be less than ideal if that data was obtained during a blink.
Traditionally, the laser surgical device operator ensures that the patient is not blinking. But, the operator may miss one or more blinks while performing other tasks during eye surgery. Hence, there is a need for a blink detection system and methods that account for a patient's blinking during eye positioning and measurement.